U.S. patent application number 14/572558 was filed with the patent office on 2015-06-18 for precision return actuator.
The applicant listed for this patent is CEDRAT TECHNOLOGIES, THALES. Invention is credited to Felix AGUILAR, Francois BARILLOT, Frank CLAEYSSEN, Christophe DEVILLIERS, Julien DUCARNE.
Application Number | 20150171308 14/572558 |
Document ID | / |
Family ID | 50624636 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150171308 |
Kind Code |
A1 |
DEVILLIERS; Christophe ; et
al. |
June 18, 2015 |
PRECISION RETURN ACTUATOR
Abstract
A nanometer-scale precision actuator comprises a base, an
intermediate structure, an output interface, and two linear
elements producing a controllable extension in the same
longitudinal direction, each between a first and a second end. A
first of the two elements has a first end fixed onto the
intermediate structure and a second end fixed onto the base, a
second of the two elements has a first end fixed onto the
intermediate structure and a second end fixed to the output
interface. The base and the intermediate structure are positioned
in such a manner that the controllable extension of the second
element produces a displacement of the actuator in a first
direction and the controllable extension of the first element
produces a displacement of the actuator in a second direction,
opposite to the first direction, with respect to the base.
Inventors: |
DEVILLIERS; Christophe;
(CANNES LA BOCCA, FR) ; AGUILAR; Felix; (CANNES LA
BOCCA, FR) ; DUCARNE; Julien; (CANNES LA BOCCA,
FR) ; BARILLOT; Francois; (MEYLAN, FR) ;
CLAEYSSEN; Frank; (MEYLAN, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THALES
CEDRAT TECHNOLOGIES |
NEUILLY-SUR-SEINE
MEYLAN |
|
FR
FR |
|
|
Family ID: |
50624636 |
Appl. No.: |
14/572558 |
Filed: |
December 16, 2014 |
Current U.S.
Class: |
310/328 |
Current CPC
Class: |
H02N 2/02 20130101; H01L
41/0986 20130101; H01L 41/12 20130101; H02N 2/06 20130101 |
International
Class: |
H01L 41/09 20060101
H01L041/09; H01L 41/12 20060101 H01L041/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2013 |
FR |
1302978 |
Claims
1. An actuator comprising: a base, an intermediate structure, an
output interface, and two linear elements producing a controllable
extension in the same longitudinal direction, each between a first
and a second end, a first of the two elements having a first end
fixed onto the intermediate structure and a second end fixed onto
the base, a second of the two elements having a first end fixed
onto the intermediate structure and a second end fixed to the
output interface, wherein the base and the intermediate structure
are positioned in such a manner that the controllable extension of
the second element produces a displacement of the output interface
in a first direction and the controllable extension of the first
element produces a displacement of the output interface in a second
direction, opposite to the first direction, with respect to the
base, and wherein the actuator comprises deformation gauges mounted
on the elements, wherein the gauges are connected so as to amplify
the effect with a view to a measurement of the extension between
the base and the output interface.
2. The actuator according to claim 1, wherein the base, the
intermediate structure, the output interface and the two elements
form several components forming a stack and wherein it comprises a
thermal regulation insert positioned between two components of the
stack.
3. The actuator according to claim 2, wherein the intermediate
structure has a U shape composed of a central part parallel to the
longitudinal direction, of a first part onto which the second
element is fixed and of a second part parallel to the first part,
substantially perpendicular to the central part, and wherein the
thermal regulation insert is positioned between the second part and
the first element.
4. The actuator according to claim 1, wherein the intermediate
structure is configured in such a manner that the thermal expansion
coefficient of the actuator has a predetermined value.
5. The actuator according to claim 4, wherein the elements are
based on piezoelectric, magnetostrictive or electrostrictive
materials.
6. The actuator according to claim 5, wherein the elements are
identical.
7. The actuator according to claim 1, further comprising a control
common to the two elements, and wherein the control is configured
so as to have a first effect on one element and a second effect,
inverse to the first effect, on the second element.
8. The actuator according to claim 7, further comprising a
closed-loop feedback of the control as a function of a measurement
of the extension between the base and the output interface.
9. Use of an actuator according to claim 7, whose common control
has an amplitude A, wherein it consists in defining a reference
position of the actuator for a control value equal to A/2.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to foreign French patent
application No. FR 1302978, filed on Dec. 18, 2013, the disclosure
of which is incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a nanometer-scale precision
actuator and can, for example, be used in the field of space
applications for an active optics application.
BACKGROUND
[0003] A telescope has a main mirror, also called primary mirror.
The primary mirror concentrates the light rays toward a secondary
mirror which reflects them back to the focal point of the
telescope. The primary mirror must not deform under the effect of
gravity for example. Often, an intermediate deformable mirror is
used to correct the defects of the primary mirror. And the
intermediate mirror is deformed by one or more actuators.
[0004] For an active optics application, an actuator with a very
high precision and stability is sought. It may even be desirable to
have a nanometer-level precision, in other words of the order of a
nanometer. Ideally, the actuator must operate at best around its
initial position. This is what is also referred to as having
symmetrical travel around the mechanical zero. Lastly, its
coefficient of expansion, denoted CTE in the literature, must be as
low as possible.
[0005] In various fields of application, precision actuators are
required. From amongst the precision actuators, piezoelectric
actuators may be mentioned.
[0006] The direct piezoelectric effect is the property according to
which the application of a mechanical load to certain crystals or
ceramics causes electrical charges to appear on the surface of the
material. The direct piezoelectric effect may be exploited in the
design of sensors such as pressure sensors.
[0007] The inverse piezoelectric effect is the property of
deformation of a piezoelectric material when an electric field is
applied to it. The inverse piezoelectric effect allows actuators to
be designed.
[0008] There exist a very large number of piezoelectric materials.
The most well known is quartz. However, it is the synthetic
ceramics PZT (for lead zirconium titanate, also known as LZT in the
literature) that are just as widely used today in the industry.
[0009] There exist two main types of piezoelectric actuators. The
first type of actuator is called a direct actuator, in which the
displacement obtained is equal to the deformation of the
piezoelectric material. Direct actuators allow a travel of between
0 and 100 micrometers to be obtained. The second type of actuator
comprises amplified actuators, in which a mechanical device
amplifies this movement by a factor of 2 to 20. Amplified actuators
generally have a travel in the range between 0.1 mm and 1 mm.
[0010] Today, it is multilayer ceramics (also known as MLA for
Multi-Layer Array in the literature) that are conventionally used
in piezoelectric actuators. The integration of this type of
material imposes specific precautions. The necessity to provide a
mechanical pre-stressing or to avoid torsion forces may in
particular be mentioned. With the proviso of a good design and
implementation, piezoelectric actuators are extremely reliable and
robust.
[0011] Their reliability and robustness have enabled piezoelectric
actuators to be used in the field of space applications. They are
also used, for example, for nanopositioning, the creation of
vibrations, and the active control of vibrations.
[0012] Today, aside from the field of space applications,
piezoelectric actuators are used in several areas. The following
may notably be mentioned:
[0013] in industry for machining assistance by creation of
vibrations;
[0014] the control of certain injectors in the automobile industry
carried out by virtue of piezoelectric materials. This technique
notably allows the process of fuel injection to be well
controlled;
[0015] some inkjet printers using piezoelectric elements for
producing the fine droplets which are propelled onto the paper.
[0016] Currently, a piezoelectric actuator with pre-stressing is
used to deform the intermediate mirror. At rest, the actuator is
said to be at its initial or reference position, also referred to
as the position of the mechanical zero. The travel of such an
actuator is asymmetric. For example, the actuator has a travel in
the range between -5 .mu.m and +40 .mu.m. The difficulty resides in
the center-shift of the travel which implies having a significant
offset of the voltage in the central position. In this case, the
initial position is no longer the desired mechanical zero.
[0017] Another solution consists in using two actuators connected
in opposition (also referred to as "push-pull" in the literature)
where their forces are added together. Each actuator must deform
the other when it is actuated. This solution only allows a limited
travel. More precisely, the actuators have a complementary
displacement. However, the asymmetry of the displacement leads to a
residual force at the mid-point or at rest. The series push-pull
doubles the force for a constant travel.
[0018] Thus, it is observed that the use of a piezoelectric
actuator alone does not allow a desired symmetrical travel to be
obtained. It is necessary to pre-stress the system and to offset
the mechanical zero. This renders problematic a potential case of
failure where the actuator gets blocked in an end position.
[0019] The use of an actuator in "push-pull" mode is a known and
advantageous solution. Nevertheless, it reduces the total travel of
the actuator and imposes the use of large actuators in order to
obtain the desired travel.
[0020] Lastly, the use of an actuator with a micromotor, a reducer
and a screw allowing a de-multiplication to be obtained is
advantageous. Nevertheless, the de-multiplication increases the
requirement in travel of the actuator which may then become too
large. This type of actuator cannot therefore be envisioned for a
space application.
SUMMARY OF THE INVENTION
[0021] The invention aims to overcome all or part of the
aforementioned problems by providing a precision actuator that can
operate around its initial position with a symmetrical travel and a
controlled coefficient of expansion.
[0022] For this purpose, one subject of the invention is an
actuator comprising:
[0023] a base
[0024] an intermediate structure,
[0025] an output interface,
characterized in that it comprises two linear elements producing a
controllable extension in the same longitudinal direction, each
between a first and a second end, a first of the two elements
having a first end fixed onto the intermediate structure and a
second end fixed onto the base, a second of the two elements having
a first end fixed onto the intermediate structure and a second end
fixed to the output interface, and in that the base and the
intermediate structure are positioned in such a manner that the
controllable extension of the second element produces a
displacement of the actuator in a first direction and the
controllable extension of the first element produces a displacement
of the actuator in a second direction, opposite to the first
direction, with respect to the base.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention will be better understood and other advantages
will become apparent upon reading the detailed description of one
embodiment presented by way of example, the description being
illustrated by the appended drawing in which:
[0027] FIG. 1 shows schematically an actuator according to the
invention;
[0028] FIG. 2 shows schematically the power supply for the
actuator;
[0029] FIG. 3a shows schematically the placement of the deformation
gauges on the actuator;
[0030] FIG. 3b illustrates the full-bridge configuration of the
deformation gauges;
[0031] FIG. 4 shows schematically an actuator according to the
invention;
[0032] FIG. 5 shows the closed-loop feedback of the control of the
actuator as a function of a measurement of extension of the
actuator.
[0033] For the sake of clarity, the same elements will carry the
same identification numbers in the various figures.
DETAILED DESCRIPTION
[0034] FIG. 1 shows schematically an actuator 10 according to the
invention. The actuator 10 comprises a base 11, an intermediate
structure 12, an output interface 13. The actuator 10 comprises two
linear elements 14, 15 producing a controllable extension in the
same longitudinal direction 50, each between a first and a second
end. A first 14 of the two elements has a first end 16 fixed onto
the intermediate structure 12 and a second end 17 fixed onto the
base 11. A second 15 of the two elements has a first end 18 fixed
onto the intermediate structure 12 and a second end 19 fixed to the
output interface 13. The base 11 and the intermediate structure 12
are positioned in such a manner that the controllable extension of
the second element 15 produces a displacement of the actuator 10 in
a first direction 51 and the controllable extension of the first
element 14 produces a displacement of the actuator 10 in a second
direction 52, opposite to the first direction 51, with respect to
the base. In FIG. 1, two elements are shown. An actuator comprising
several other elements, for example 3, 4 or more, could perfectly
well be envisioned.
[0035] The base 11, the intermediate structure 12, the output
interface 13 and the two elements 14, 15 form several components
forming a stack. Thus, when the element 15 is extended, it moves
the stack in the direction 51. When the element 14 is extended, it
moves the intermediate structure 12 in the direction 52. An
actuator capable of having a symmetrical travel around its initial
position is thus obtained.
[0036] Furthermore, the intermediate structure 12 may take various
forms. What may notably be distinguished is an intermediate
structure referred to as axial with elements superposed in the
longitudinal direction (as is the case in FIG. 1) of an
intermediate structure referred to as lateral such as for the
actuator 100 shown in FIG. 4. The actuator 100 is identical to the
actuator 10 except that the actuator 100 comprises three elements
41, 42 and 43. The elements 41, 42 and 43 have a controllable
extension in the longitudinal direction 50. They are positioned one
next to the other parallel to the longitudinal direction 50. The
element 41 has a controllable extension in the direction 51 and the
extension of the elements 42 and 43 moves the actuator 100 in the
direction 52. In FIG. 4, three elements are shown. The actuator 100
could comprise several others of them. For example, the element 41
could be replaced by two elements. An actuator with an intermediate
structure referred to as axial occupies a larger space in the
longitudinal direction, whereas an actuator with an intermediate
structure referred to as lateral occupies a smaller space in the
longitudinal direction but takes up more space in the lateral
direction. In practice, the shape of the intermediate structure 12
is chosen according to the space that may be occupied by the
actuator 10 within its environment.
[0037] The intermediate structure 12 is configured in such a manner
that the thermal expansion coefficient of the actuator 10 has a
predetermined value. Indeed, by choosing the thicknesses and
materials of the intermediate structure 12 appropriately, a stack
can then be obtained that is referred to as athermal. An expansion
of the actuator 10 subjected to a given rise in temperature is now
considered. When expanding, the element 15 gets longer by a
distance e in the direction 51. By choosing elements 14 and 15 that
are identical or at least similar, the element 14 expands in the
same way, in other words the element 14 also expands by a distance
e. However, since the element 14 is positioned between the base 11
and the intermediate structure 12, the expansion of the element 14
results in a displacement of the intermediate structure 12 by a
distance e in the direction 52. The expansion of the element 15 and
the expansion of the element 14 compensate for each other.
[0038] Aside from the elements 14 and 15, other components of the
stack may be formed from materials whose coefficients of expansion
are as low as possible. The intermediate structure 12 and the base
11 are for example composed of an alloy of iron (64%) and nickel
(36%). This alloy has a very low coefficient of expansion
(1.2.times.10.sup.-6 K.sup.-1). The intermediate structure 12 and
the base 11 may also be made of ceramic, for example silicon
nitride. The output interface 13 can be made of glass of
vitro-ceramic type and can also have a very low thermal expansion
coefficient.
[0039] Components of the stack having higher coefficients of
expansion may be chosen while at the same time conserving an
overall thermal expansion coefficient for the stack of zero. It is
also possible for the overall thermal expansion coefficient of the
stack to be zero by inserting intermediate components. The
intermediate structure 12 can have a U shape composed of a central
part 60 parallel to the longitudinal direction 50, of a first part
61 onto which the second element 15 is fixed and of a second part
62 parallel to the first part 61, substantially perpendicular to
the central part 60.
[0040] In the case of expansion of the intermediate structure 12,
it is the part 60 that expands in a significant manner in the
longitudinal direction 50. The element 15 therefore moves with the
expansion of the intermediate structure 12. The actuator 10 may
comprise a thermal regulation insert 20 positioned between two
components of the stack. Advantageously, the thermal regulation
insert 20 is positioned between the second part 62 and the first
element 14. In other words, the insert 20 is positioned between the
first end of the first element 14 and the intermediate structure
12. The insert 20 may be formed so as to adjust the length of the
central part 60 of the intermediate structure 12. The insert 20 may
be formed from a material with a high thermal expansion
coefficient. The insert 20 then expands as much as the intermediate
structure 12 and moves the intermediate structure 12 in the
direction 52 so as to counter-balance the displacement in the
direction 51 due to the expansion of the intermediate structure 12.
Placed between the element 14 and the intermediate structure 12,
the insert 20 allows an overall thermal expansion coefficient of
the stack of zero to be obtained. The adjustment of the height of
the insert 20 regulates the coefficient of expansion of the stack
both in the increasing and in the decreasing direction.
[0041] In operation, the element 14 extends at its second end; it
is said to pull on the intermediate structure 12. The element 15
mounted on the intermediate structure 12 extends at its second end;
it is said to push the actuator. Mechanically, each of the two
elements 14, 15 provides half of the travel; this is what is
referred to as a "return" operation. An actuator 10 is thus
obtained with a symmetrical travel.
[0042] When the actuator 10 operates in an environment subjected to
variations in temperature, the two elements 14, 15 expand. The
return operation allows the output interface 13 of the actuator to
remain fixed. In other words, the actuator 10 is insensitive to a
simultaneous expansion of the elements, a fact which endows it with
a positioning precision.
[0043] The elements 14, 15 may be based on piezoelectric,
magnetostrictive or electrostrictive materials.
[0044] The elements 14, 15 may be equipped with flexible guiding
elements allowing the actuator 10 to be rigidified.
[0045] The actuator 10 can be used with a travel amplification. The
travel amplification is achieved thanks to a mechanical device to
which the actuator 10 is connected by use of a lever arm. The
travel amplification allows a longer travel to be obtained to the
detriment of the rigidity and of the precision.
[0046] Advantageously, the elements 14, 15 are identical.
[0047] FIG. 2 shows schematically the power supply for the actuator
10. The actuator 10 comprises a control 21 common to the two
elements 14, 15. The control 21 is configured so as to have a first
effect on one element and a second effect, inverse to the first
effect, on the second element. The inversed-effect control 21 is
for example carried out by means of three voltages V0, Vcc and
Vcom. The voltages V0 and Vcom are fixed and the voltage Vcom
varies between V0 and Vcc. The element 14 is controlled between the
voltages V0 and Vcom. The element 15 is controlled between the
voltages Vcom and Vcc. Thus, the voltage across the terminals of
the element 15 is Vcc-Vcom and the voltage across the terminals of
the element 14 is Vcom-V0. The common control 21 has an amplitude A
close to Vcc-Vcom. A reference position, also called initial
position or mechanical zero, of the actuator 10 is defined for a
control value equal to A/2. The common control 21 acts on Vcom
which will generate a positive voltage variation (respectively
negative) between the terminals of the element 14 (respectively 15)
and vice versa. Under the effect of this voltage variation across
the terminals of each of the elements 14, 15, this results in an
extension (respectively retraction) of one of the two elements. In
other words, by varying Vcom negatively for example, the voltage
across the terminals of the element 15 increases. The element 15
extends in the longitudinal direction 50 in the direction 51 and
thus moves the output interface by a distance d/2. By varying Vcom
positively, the voltage across the terminals of the element 14
increases. The element 14 extends in the longitudinal direction 50
in the direction 51 and, since the second end of the element 14 is
fixed onto the base 11, thus moves the intermediate structure 12,
in other words the actuator 10 is moved by a distance d/2 in the
longitudinal direction 50 in the direction 52. The travels of each
element are added together. The actuator 10 therefore has a travel
equal to d. In practice, long travel lengths may be obtained, of
the order of around thirty micrometers. The travel thus obtained
will generate a displacement of the actuator 10 allowing the
intermediate mirror (not shown in FIG. 2) to be deformed in the
desired direction and with the desired amplitude.
[0048] The common control 21 allows a good linearity around the
mechanical zero to be obtained with the amplitude A/2, thus
ensuring a high precision (of the order of a nanometer) and a high
stability. Indeed, in the case of a separate power supply for the
elements, and hence of a separate control, it is necessary to
switch from one control to another, a fact which renders the
control more complex.
[0049] Furthermore, the common control 21 simplifies the
implementation of such an actuator and is particularly beneficial
in the case where several actuators are used.
[0050] Finally, in the case of a failure, for example if the
actuator 10 is no longer powered, the common control 21 is
particularly advantageous. The actuator 10 remains in its reference
position at the mechanical zero, whereas in the case of an actuator
having for example a travel in the range between -5 .mu.m and +40
.mu.m with a separate power supply, the actuator gets blocked in an
end position.
[0051] FIG. 3a shows schematically one example of placement of four
deformation gauges on the actuator 10. The actuator 10 comprises
deformation gauges mounted on the elements 14, 15. The elements 14,
15 each comprise two deformation gauges which are deformed
according to the extension of the elements. The element 14
comprises two deformation gauges 32, 34 and the element 15
comprises two deformation gauges 31, 33. The gauges are connected
so as to amplify the effect with a view to a measurement of the
extension between the base 11 and the output interface 13. The
gauges 31, 32, 33, 34 allow the deformation of the elements 14 and
15 in the longitudinal direction 50 to be measured.
[0052] It is to be noted that a simplified configuration using one
gauge may also be envisioned. This case is known as a half-bridge
configuration. However, this configuration is sensitive to the
bending of the actuator.
[0053] A deformation gauge is a very fine resistant wire printed or
adhesively bonded onto an insulating medium placed on the element
whose deformation it is desired to quantify. When the medium is
deformed, the wire is stretched. Its electrical resistance then
varies in proportion to the variation in length. By measuring the
resistance variation, its deformation is deduced, and consequently
the deformation of the element. In order to transmit the
deformations of the element as faithfully as possible, the medium
carrying the gauge must have very specific characteristics. A good
aptitude for adhesive bonding, a low coefficient of expansion and
also an ability to withstand temperature variations may for example
be noted.
[0054] The variations in resistance of the deformation gauges are
too small to be directly measurable. The deformation gauges are
consequently assembled according to a full-bridge electrical
configuration which allows the variation in resistance to be
accessed. In FIG. 3a, the deformation gauges 31, 32, 33, 34 are
placed on the elements 14, 15 under the same adhesive bonding
conditions.
[0055] FIG. 3b illustrates the full-bridge configuration of the
deformation gauges 31, 32, 33, 34. The gauge 31 is situated between
two points 101 and 102, the gauge 32 is situated between two points
102 and 103, the gauge 33 is situated between two points 103 and
104 and the gauge 34 is situated between the points 104 and 101.
The gauges 31 and 32 are connected in series between the points 101
and 103. Similarly, the gauges 33 and 34 are connected in series
between the points 101 and 103. The voltage across the terminals of
the gauges 31 and 32 connected in series, which is the same as the
voltage across the terminals of the gauges 33 and 34, is referred
to as the supply voltage for the bridge.
[0056] A full-bridge configuration allows an optimum sensitivity to
be obtained. There is no bias in the measurement of the extension
of the elements, the bridge is only sensitive to the useful
deformations. In other words, in case of thermal deformation of the
elements 14 and 15, the four gauges see their resistance being
modified simultaneously and in the same direction. When such a
modification occurs, the output voltage of the bridge remains
unchanged. The gauge bridge is connected to a comparator 72 which
powers the sensor, processes and amplifies the value of the
measurement.
[0057] FIGS. 3a and 3b also show the power supply of the actuator
10 by a power supply circuit 71. Two wires 73, 74 power the bridge.
Two other wires 75, 76 return the unbalance of the bridge to the
comparator 72 when the electrical resistances of the gauges vary.
Two other wires 77, 78, also called return wires, allow a
measurement of the supply voltage between the points 101 and 103
which is returned to a comparator 70. The comparator 70 then acts
on the power supply circuit 71 so as to keep the power supply
voltage constant. This configuration with 6 wires allows the
in-line losses that can occur because of the resistance of the
cabling to be eliminated.
[0058] FIG. 5 shows the closed-loop feedback of the control of the
actuator 10 as a function of a measurement 90 of extension of the
actuator 10 coming from the comparator. The actuator 10 comprises
an active closed loop between the common control 21 to the two
elements 14, 15 and the measurement 90 of the extension of the
elements. In order to deform the intermediate mirror in the desired
direction, an initial command 80 indicates to the power supply
circuit 71 to power the bridge. The elements 14, 15 are deformed in
the longitudinal direction 50 in the desired sense. A deformation
85 in the longitudinal direction 50 of the elements 14, 15 can be
quantified by a measurement 90 of the deformations by virtue of the
deformation gauges. This measurement 90 can be returned to a
comparator 70 which compares the measured value to the initial
setpoint value and adjusts the power supply voltage of the bridge
accordingly. The actuator 10 comprises a closed-loop feedback of
the control 21 as a function of a measurement of the extension
between the base 11 and the output interface 13.
[0059] In the case where there is no control 21 common to the two
elements 14 and 15, in other words if the elements 14 and 15 have a
separate control, the actuator 10 then comprises one loop per
element.
[0060] The actuator 10 according to the invention thus disposes of
a large travel that is symmetrical around its reference position.
It is athermal. It has a simple control and closed-loop feedback.
Lastly, it occupies a reasonable volume while at the same time
having robustness and lifetime characteristics compatible with a
use in the field of space applications.
* * * * *